Measurement of voltage and current for estimation of power is a fundamental problem of power sensing. One prior art device uses a split-core transformer which momentarily opens to enclose a single conductor, which causes a magnetic flux to be developed in the split-core transformer. Making a magnetic flux measurement using the split core transformer, it is thereby possible to estimate the current flowing through the conductor enclosed by the split-core transformer. A voltage measurement may be combined with the current measurement to form a power measurement, such as by multiplying the current and voltage and computing a root mean square (RMS) value to generate the RMS power estimate.